The invention relates to the field of tone recognition in telecommunications and more particularly to the detection of particular sequences of tones used to indicate the initiation of a fax transmission.
It has become common in voice communication networks to employ voice compression technology in order to increase the number of connections that may be carried on a limited number of lines in a network. Because a human voice signal contains significant redundancy, digitized voice communications are readily compressible using any of a number of available voice compression algorithms. Although some signal quality is lost when a voice transmission is compressed, the losses in quality are usually minimal and offset by the lower costs derived from the use of more voice paths on fewer lines. Voice compression ratios in the range of 4:1 and 8:1 and higher have become common in telephone transmission, and though quality of the voice signal may be slightly compromised at these ratios, the quality remains high enough for effective communication.
Most communication networks which carry voice transmissions are also used for carrying data transmission. The most common example of a network which carries both voice and data transmissions is the general switched telephone network which carries voice, fax and modem transmissions. Because data transmissions such as fax and modem contain different information redundancy than voice transmissions, data transmissions are not properly propagated through compressed voice channels. Fax and modem transmissions may however be compressed using compression techniques specifically designed for such transmissions. The extent to which voice compression will degrade a fax transmission depends on a number of factors, including the speed of the fax transmission, the sophistication of the voice compression algorithm and the compression ratio. For example a slow (eg. 2400 bits per second) fax transmission could succeed in being properly exchanged on a voice compression channel, but higher bit rate transmissions like 14.4 kbps would fail.
Therefore, in order for a mixed voice/data communication network to support high voice compression rates, fax transmissions must be detected and routed over uncompressed lines or fax relays so that the fax transmissions are not propagated through the voice compression system.
Under International Telecommunication Union Telecommunication Standard Sector""s (ITU-T) Recommendation T.30, fax transmissions are initiated by the transmission of a ITU-T V.21 300 bit per second (b.p.s.) FSK (frequency-shift-keying) modulated signal. The initial sequence of the ITU-T V.21 signal consists of the repeated transmission of many HDLC (High level Data Link Control) IDLE flags each consisting of the binary sequence 01111110. In the V.21 channel 2 FSK modulation, 1""s are represented by a 1650 Hz tone and 0""s are represented by a 1850 Hz tone. Thus, the initiation sequence of the fax transmission is indicated by an initial tone at 1850 Hz for a duration of 3.3 milliseconds (corresponding to one 0 at 300 b.p.s.) followed by a tone at 1650 Hz for a duration of 20 milliseconds (corresponding to the six 1""s at 300 b.p.s.) followed by a tone at 1850 Hz for a duration of 6.6 ms (corresponding to the two 0""s at 300 b.p.s.). Thereafter an alternating sequence of tones at 1650 Hz for 20 ms and at 1850 Hz for 6.6 ms is repeatedly transmitted.
The T. 30 protocol also allows for the sharing of a single 0 bit between two adjacent HDLC IDLE flags. Therefore, it is also possible to encounter a 3.3 ms tone at 1850 Hz between two 20 ms tones at 1650.
Two methods of detecting V.21 fax transmissions are common in the prior art. The first method involves using a tone detector tuned to 1650 Hz. If the tone detector tuned to 1650 Hz detects a predominant 1650 Hz tone in the incoming signal for a particular period of time then a fax transmission is assumed to have been detected and is routed to an uncompressed line or fax relay. The tone detectors used in the prior art are typically band pass filters tuned to 1650 Hz. If a predominant portion of the signal passes through the filter for a particular period of time, then this prior art method assumes that the 1650 Hz tone is predominant because the signal is carrying the initiation sequence of a V.21 fax transmission. This prior art method has several drawbacks including possible false detections which can be triggered by music or sibilance in a voice signal or detection of crosstalk from a fax transmission on another line. In addition, the necessary components for the filters used in this method are relatively expensive. However, one advantage of this technique is that it may be implemented using a relatively small number of operations in a computer.
A second prior art method for detecting a fax transmission uses a V.21 demodulator to perform a full demodulation and conversion of the signal into a bit stream. The bit stream is then analysed for repetition of the specific HDLC bit sequence (01111110) which indicates the beginning of a fax transmission. This method of detection accurately detects fax transmissions and reduces the probability of false detections encountered in the first method. However, compared to tone detection this method is computationally complex requiring significantly more computer operations making it more expensive and complex to implement on a system.
It can be seen that a method of detecting a fax transmission which is both accurate and simple, requiring fewer computer operations than a full V.21 demodulation, is desirable.
According to a first broad aspect of the present invention, there is provided a method for recognizing a tone having a tone frequency and a wave shape in a signal in a telecommunications transmission, the method including the steps of sampling the signal at a sampling frequency to obtain first and second consecutive samples of the signal; obtaining a predicted value corresponding to a third consecutive sample of the signal using the first consecutive sample, the second consecutive sample, the tone frequency, the wave shape and the sampling frequency; sampling the signal at the sampling frequency to obtain the third consecutive sample of the signal; deriving a prediction error indicative of the presence of the tone in the signal using the third consecutive sample and the predicted value; and testing the prediction error against a threshold value to determine whether the signal contains the tone.
According to a second broad aspect of the present invention, there is provided a method for recognizing a sequence transmitted in a signal, the sequence having a plurality of tone intervals each having an interval frequency and an interval duration, the method including the steps of: detecting a tone in the signal having a frequency matching the interval frequency of a tone interval of the sequence; timing the tone to obtain a timed duration; detecting a subsequent tone having a frequency matching the tone frequency of a subsequent tone interval of the sequence if the timed duration of the previous tone matches the interval duration of the previous tone interval; timing the subsequent tone in order to obtain a subsequent timed duration; and repeating the previous two steps to detect and time subsequent tones until, for every tone interval in the sequence, a tone is detected having a frequency and a timed duration matching the interval frequency and the interval duration of a corresponding tone interval.
According to a third broad aspect of the present invention, there is provided a method for recognizing a fax transmission in a signal, the fax transmission having an initiation sequence for indicating the beginning of the fax transmission, the initiation sequence having a plurality of tone intervals each having an interval frequency, an interval duration and a wave shape, the device including a tone detector for recognizing tones in the signal having frequencies corresponding to the interval frequencies of the tone intervals in the initiation sequence of the fax transmission, and a sampler for sampling the signal at a sampling frequency to generate digital samples of the signal; wherein consecutive samples are provided to the tone detector and the tone detector uses two consecutive samples, the sampling frequency, the interval frequency and the wave shape to obtain a predicted value of a third consecutive sample, wherein a prediction error indicative of presence of a particular tone in the signal is derived using the third consecutive sample and the predicted value and wherein the prediction error is tested against a threshold value to determine whether the signal contains a tone having a frequency corresponding to the interval frequency.
According to a fourth broad aspect of the present invention, there is provided a device for recognizing a modem transmission in a signal, the modem transmission having an initiation tone for indicating the beginning of the modem transmission, the initiation tone having an initiation frequency and a wave shape, the device including a sampler for sampling the signal at a sampling frequency to generate digital samples of the signal, and a tone detector for recognizing a tone in the signal having a frequency matching the initiation frequency, wherein consecutive samples are provided to the tone detector and the tone detector uses two consecutive samples, the sampling frequency, the initiation frequency and the wave shape to obtain a predicted value of a third consecutive sample, and wherein a prediction error indicative of the presence of the initiation tone in the signal is derived using the third consecutive sample and the predicted value, and wherein the prediction error is tested against a threshold value to determine whether the signal contains a tone having a frequency corresponding to the initiation frequency.